This invention relates generally to a reactor for depositing an epitaxial layer on a semiconductor wafer, and more particularly to a reactor including an injector having slots for metering reactant gas flow through the reactor.
Conventional horizontal reactors have a reaction chamber extending horizontally between an entrance and an exit for admitting and venting reactant gas to and from the chamber, respectively. During the chemical vapor deposition process, a semiconductor wafer is positioned on a rotatable susceptor positioned in the reaction chamber, and reactant gas is delivered to the chamber through an inlet passage connected to the entrance of the chamber. The flowrate of the reactant gas passing over the wafer is critical to the thickness profile and electrical properties of the epitaxial layer formed on the wafer during the chemical vapor deposition. To ensure the desired properties are achieved, the gas is metered by an injector positioned along the inlet passage.
The injector includes a metering fixture having one or more large openings through which the gas passes when traveling to the reactor. Blades or metering plates fastened across the openings in the metering fixture partially block the openings and form narrow slots through which the reactant gas flows. These slots limit the flowrate of the reactant gas delivered to the reaction chamber. Each blade is held in place with screw fasteners so its position is adjustable relative to the openings. As the blade position is adjusted, the flow area and thus the flowrate through the slots changes. The blade positions may be adjusted so the slots upstream from the center of the reaction chamber are wider than those upstream from the sides of the chamber. This results in more gas passing over the center of the wafer to increase the thickness of the epitaxial layer at the center. If the slot widths corresponding to the sides and center of the reaction chamber are equal, an undesirable concave epitaxial surface results.
The reactant gas flowrate is a function of the width of the slots. More particularly, the flowrate through each slot is a function of the cube of the width of the slot. As a result of this relationship, the gas flowrates through the slots are highly dependent upon their widths. For instance, if a slot width is 0.004 inches when 0.003 inches is desired, the flowrate through the slot will be nearly 240 percent of the desired flowrate. Thus, it may be seen that precise control of the slot width is required to produce the desired epitaxial layer. Presently, such control is difficult to achieve. The blades can shift as the screw fasteners holding them are tightened, causing deviations from the desired in slot width and thus flowrate.
Moreover, because the slot is formed between the edges of the blade and the opening in the metering fixture, gas passing through the slot travels along the wall of the opening. As a result, flow downstream from the injector is easily disturbed by discontinuities in the wall of the inlet passage. Such flow disturbances can negatively affect the quality of epitaxial layers produced by the reactor.
In order to alleviate large flowrate variations due to set-up variability and the potential for flow disturbances, another type of blade which is sized to entirely cover the injector openings has been conceived. These blades have a series of holes drilled through them to allow reactant gas to pass through the blades. Although these blades eliminate flowrate variation caused by set-up variability, they increase flowrate variation caused by the machining tolerances of the holes in the blades. Flow through a circular hole is a function of its diameter to the fourth power. Thus, the flowrate through a 0.004 inch diameter hole is about 320 percent of the flowrate through a 0.003 inch diameter hole. As will appreciated by those skilled in the art, large flowrate variations can result from small discrepancies in the injector hole diameters. Moreover, since the hole diameters are fixed, the blades must be replaced rather than adjusted if different flowrates are desired.